I know this would sound somewhat controversial, but a Mass Effect like universe could support the variety of species and "planes" that we could see in PsT. Or some other sci-fi universe, where a multiverse becomes available through technology instead of magic (or creation). One could even bring in worm-holes and weird time-paradoxes.

I don't see why a spiritual successor has to be fantasy, it could easily be sci-fi.
I mean he even said that he doesn't want to have it in a classic fantasy world. It might even be easier to create it in sci-fi or a mixture of sci-fi/fantasy.